Two different approaches have traditionally been used for preventing undesirable vortex-induced vibrations. A first approach uses so-called strakes, i.e. helically arranged continuous low-profile fins arranged along the pipe being subjected to the water current. Strakes are made with different helical pitch and with single, double-start or triple-start helices about the pipe. A pipe provided with strakes may have a significantly reduced cross-flow displacement compared to the naked pipe in the water current. However a considerable disadvantage of strakes is that they significantly increase drag and may thus increase the vertical and horizontal load on the riser system, the mooring system and the platform.
Another approach for preventing vortex-induced vibrations comprises several types of so-called fairings, i.e. teardrop-profiled wings arranged more or less concentrically about the pipe in order to reduce the drag force and to partially prevent vortex-induced vibrations.
Passively rotatable fairings are used for reducing drag and counteracting vortex-induced vibrations on a riser. A considerable disadvantage related to the use of traditional fairings is that, in order to effectively prevent vortex-induced vibrations, the chord length-to-diameter ratio must be high, and thus the drag increases. Further, if such long-tail fairings are arranged, the weight load on the riser and thus on the suspending platform may increase prohibitively. Long-tail fairings may also be expensive to install and to maintain.
Additionally, passively rotatable fairings have been shown to be unstable under certain circumstances, such as high relative water speed combined with relatively short chord length-to-diameter ratios of the fairing. The instability in mention resembles so-called “galloping” or “fluttering”, terms used in civil and marine engineering and aerospace literature. The purpose of the present invention is to prevent such fairing instability.
An Offshore Technology Conference paper, OTC 16342 “Flutter instability in Riser Fairings”, S. T. Slocum et al, 2004, describes experiments involving riser fairing models designed to rotate freely about a pipe axis and to passively align with the direction of incident flow so as for streamlining the flow regardless of current direction. It is observed that the rotational degree of freedom incurs the disadvantageous effect of coupling between cross-flow translation and rotation of the fairing and the pipe.
Several fairings are known from published patents and patent applications.
U.S. Pat. No. 5,410,979 describes relatively small fixed teardrop fairings for vortex induced vibration suppression. The fairings are described as non-rotatable. Accelerometers are mentioned in col. 3, line 39-47 for measuring accelerations of a model of a riser model provided with fairings. The measurements are given as a RMS acceleration for varying angles of attack of the water current relative to the fairing's fixed direction. U.S. Pat. No. 5,410,979 further mentions U.S. Pat. No. 4,398,487 and U.S. Pat. No. 4,474,129 describing passively rotatable fairings.
U.S. Pat. No. 4,474,129 defines a fairing constructed of synthetic foam and formed in two symmetrical parts that may be removably secured to riser buoyancy modules. The fairing has a neutral buoyancy in water.
U.S. Pat. No. 4,398,487 defines a rotatable fairing with bearings arranged for being connected to a vertical cylindrical element such as a flotation module on a marine drilling riser. The fairing comprises a heading or nose portion having a vertically arranged central bore for the generally cylindrical buoyancy element of the riser. The semicylindrical surface of the heading portion forms an even transition symmetrically on either side of the riser's axis to a tail portion arranged for turning passively with the water current flow. Tail fins are arranged at the trailing end of the tail portion and helps the fairing align with the current flow.
U.S. Pat. No. 6,067,922 describes a fairing manufactured from a simple plate that is bent around the riser pipe and of which the tail-forming plates are joined using attachment bolts and arranged for weathervaning to orient effectively with the water current.
U.S. Pat. No. 6,179,524 describes fixed fairings arranged with different orientations about a current direction expected to be a prevailing current direction.
U.S. Pat. No. 6,196,768 describes a rotatable fairing for an entire hull of a spar buoy platform. The fairing is provided with neutral buoyancy. Col. 4, lines 7-15 describes that the fairing may be rotated by using mooring lines.
U.S. Pat. No. 6,223,672 mentions fixedly mounted fairings and mentions the same U.S. Pat. No. 4,398,487 and U.S. Pat. No. 4,474,129 as mentioned above, in which is claimed in col. 1, line 66, that “Further, the subsea environment in which the fairings must operate renders likely the rapid failure of the rotational elements”. Thus rotatable fairings are considered as unreliable in the prior art.
U.S. Pat. No. 6,401,646 describes a “snap-on” fairing.
U.S. Pat. No. 6,415,730 shows a fairing having several depressions in the surface for reducing the water's friction with the fairing surface, thus for reducing the drag in water.